1. Field of the invention
The present invention is generally concerned with lifting winches.
It is more particularly concerned with drilling rigs.
2. Description of the prior art
The lifting winch on a drilling rig is conventionally driven by a unidirectional diesel engine or DC electric motor coupled to a clutch, a gearbox and a brake for stopping the string of drilling pipes during raising or lowering and for controlled retention during lowering or drilling.
A water cooling circuit is usually provided for the brake, which is usually a belt type friction brake, and a hydraulic or electromagnetic retarder is also usually provided.
Apart from the resulting area of the drilling rig deck taken up by the system and the inevitable additional noise due to the friction brake, the major disadvantage of this drive method is that in practice it requires continuous control over the parameters of the movement being carried out including distance, speed, acceleration and force.
In particular, the operator is responsible for keeping to the authorized maximum speeds during raising and lowering and for halting the string of drilling pipes at the end of upward or downward movement at positions that are specified to a greater or lesser degree, whilst also monitoring the various dials on the control panel.
Given these conditions, mishaps are by no means rare, especially at the ends of travel.
To minimize this risk it is usual practice to provide generous stopping distances as a safety measure, especially at the top. This increases the height of the derrick and the operator, working by sight, naturally tends to start the slowing process each time sooner than is strictly necessary, which is detrimental to productivity.
This problem is accentuated by the fact that when an electromagnetic retarder is used it is also necessary to allow for its non-negligible response time.
French Pat. No. 2 559 540 proposes to use sensors for automatically controlling the raising travel of the string of drilling pipes so that it can be systematically stopped in an accurate manner at the end of travel under optimum conditions.
However, the motor driving the lifting winch is a conventional unidirectional motor associated with a friction brake and an electromagnetic slowing device.
In "Transmissions diesel-electriques dans le forage petrolier" ("Diesel-electric transmission systems for oil drilling applications") by G. PASTERNAK published by TECHNIP in 1970, it is proposed to substitute for the unidirectional motor an electric motor able to operate in all four quadrants of the torque-speed diagram.
This of course presupposes the provision of control means for controlled excitation of the electric motor with, as is standard practice in controlling electric motors, a manipulator disposed on an operator control panel and positioned to establish a given speed or displacement request instruction addressed to the control means.
As used here the term "excitation" is to be understood in a general sense as referring to all aspects of supplying electric power to an electric motor, without necessarily being limited, for example, to the energization of the rotor in the case of a synchronous motor or to the energization of the stator in the case of a DC motor.
There is no utility in providing a clutch, a conventional type gearbox, a friction brake or an electromagnetic retarder because the electric motor used is inherently able to regulate the speed and position of the string of drilling pipes directly, both during raising, when it actually functions as a motor, and during lowering, when it functions as a generator, with the possible exception of an initial acceleration phase.
A general object of the present invention is a control device for lifting winches, in particular for a drilling rig, with the advantage of enabling use of this arrangement.